1. Field of the Invention
The present invention relates to a structure of a junction between a copper ball and an aluminum electrode when a copper wire is employed.
2. Description of the Related Art
FIG. 1 is a cross-sectional view illustrating a conventional structure of wire bonding of a copper wire, while FIG. 2 is a diagram illustrating the distribution of aluminum after a wire-bonded copper ball and a layer of copper-aluminum alloy are etched with nitric acid.
In these diagrams, a layer of a copper-aluminum alloy (hereinafter simply referred to as an alloy layer) 3 is produced under a copper ball 1 at the time of bonding, and this alloy layer 3 separates aluminum 21 of an electrode 2 underneath the copper ball 1 from aluminum 22 around the copper ball 1. An insulating layer 4, which is formed of an oxide, nitride, or the like, constitutes a substrate of the aluminum electrode 2.
In FIGS. 1 and 2, the alloy layer 3 is produced by ultrathermosonic compression bonding, that is the copper ball 1, which is formed at the end of the copper wire using heat energy, is pressed against the aluminum electrode 2 which is heated to 300.degree.-400.degree. C. This alloy layer 3 has a mechanical strength which is greater than that of the copper ball 1. At this time, the aluminum electrode 2 is plastically deformed by the pressure of the copper ball 1, so that the aluminum electrode 2 is separated into the aluminum 21 underneath the copper ball 1 and the aluminum 22 around the copper ball 1. Consequently, the electrical connection to the device is obtained only at the junction between the ally layer 3 and the aluminum 22 around the copper ball 1. Incidentally, this contact and the associated device are subsequently encapsulated with an epoxy-resin.
Thus, in the conventional structure of an electrode junction for a semiconductor device, since the electric connection is obtained only at the junction between the alloy layer 3 and the aluminum 22 around the copper ball 1, there has been a problem. If the circumferential portion of the alloy layer 3 bonded to the aluminum 22 around the copper ball 1 deteriorates even slightly, the electrical resistance of the contact exceeds a maximum acceptable value and ends the life of the device.